1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a liquid crystal display with a gate line structure that can serve as a storage electrode and a black matrix and perform a repair function.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmissivity using an electric field to display a picture. To this end, the LCD includes a liquid crystal panel having liquid crystal cells arranged in a matrix type, and a driving circuit for driving the liquid crystal panel. The liquid crystal panel is provided with pixel electrodes for applying an electric field to each liquid crystal cell, and a reference electrode, that is, a common electrode. Typically, the pixel electrodes are provided on a lower substrate for each liquid crystal cell, whereas the common electrode is integrally formed on the entire surface of an upper substrate. Each of the pixel electrodes is connected, via source and drain terminals of a thin film transistor (TFT) used as a switching device, to any one of data lines. A gate terminal of each TFT is connected to any one of gate lines for applying a pixel voltage signal to pixel electrodes for one line.
FIG. 1 shows a thin film transistor substrate for a conventional liquid crystal display (LCD). The LCD includes thin film transistors 6 positioned at intersections between data lines 2 and gate lines 4, and pixel electrodes 14 connected to drain electrodes 12 of the thin film transistors 6. The thin film transistor 6 is provided at an intersection between the data line 2 and the gate line 4. The thin film transistor 6 has a gate electrode 10 connected to the gate line 4, a source electrode 8 connected to the data line 2, and a drain electrode 12 connected, via a first contact hole 16, to the pixel electrode 14.
The thin film transistor 6 further includes a semiconductor layer (not shown) for providing a conductive channel between the source electrode 8 and the drain electrode 12 by a gate voltage applied to the gate electrode 10. The thin film transistor 6 responds to a gate signal from the gate line 4 to selectively apply a data signal from the data line 2 to the pixel electrode 14. The pixel electrode 14 is positioned at a cell area divided by the data line 2 and the gate line 4, and is made from an indium tin oxide (ITO) material having high light-transmissivity. The pixel electrode 14 generates a potential difference from a common transparent electrode (not shown) provided at an upper glass substrate by a data signal applied via the first contact hole 16. By virtue of this potential difference, a liquid crystal positioned between the thin film transistor substrate and the upper substrate is rotated according to its dielectric anisotropic property and a light applied, via the pixel electrode 14, from a light source is transmitted into the upper glass substrate.
A storage capacitor 18 provided between the pixel electrode 14 and the gate line 4 at the previous stage plays a role in preventing voltage variation in the pixel electrode 14 by charging a voltage in a period at which a gate high voltage is applied to the previous-stage gate line 4 and discharging the charged voltage in a period at which a data signal is applied to the pixel electrode 14. Since, as stated, the storage capacitor 18 aims at maintaining a stable pixel voltage, it must have a high capacitance value. To this end, the storage capacitor 18 has a structure as shown in FIG. 2.
In FIG. 2, the storage capacitor 18 is defined by a storage electrode 20 electrically connected, via a second contact hole 22 formed in a protective film 28, to the pixel electrode 14 and a gate electrode 4 having on a gate insulating layer 26 therebetween. The storage electrode 20 is formed on the gate insulating layer 26 upon formation of the data line 2 and the source/drain electrodes 8 and 12. As a liquid crystal panel goes into a larger dimension, the capacitance value of the storage capacitor 18 must be increased. However, the above-mentioned LCD structure is limited in its ability to enlarge the capacitance of the storage capacitor 18.
The protective film 28 of the thin film transistor substrate is usually made from an inorganic material having a dielectric constant such SiNx or SiOx. The pixel electrode 14 and the data line 2 having such an inorganic protective film therebetween maintain a certain horizontal distance d (e.g., 3 to 5 μm), as shown in FIG. 3, so as to minimize any coupling effect caused by a parasitic capacitor. In this case, in order to shut off light leaking through the space between the data line 2 and the pixel electrode 14, a black matrix formed on the upper substrate has a width sufficient to cover a portion of the pixel electrode 14 positioned at each side of the data line 2. As a result, the aperture ratio of the liquid crystal cell is inevitably reduced.